1. Field of the Invention
The present invention relates to a nitride semiconductor free-standing substrate and a device using the same and, in particular, to a nitride semiconductor free-standing substrate having a surface inclined at a predetermined angle to a C-plane and a device manufactured using the same.
2. Related Art
A nitride semiconductor has attracted attention as a blue, green and ultraviolet light-emitting device material or a high-output electronic device material.
As for a nitride semiconductor of which device application has been reported, almost all of them are conventionally realized, for example, by growing a nitride semiconductor on a hetero-substrate such as a sapphire, 4H—SiC or the like via a low temperature GaN or AlN buffer or a high temperature AlN buffer.
The surface of these substrates is generally inclined at about 0.1° to 10° to C-plane ((0001) plane). Since molecular steps (hereinafter referred to as simply “step”) are regularly arranged on the surface by inclining the substrate surface to the C-plane, atoms supplied onto the substrate surface are likely to be incorporated into the steps when growing a nitride semiconductor crystal on the substrate, which results in so-called step-flow mode growth, and a good quality crystal with few defects can be obtained.
As an off-orientation of such substrates, namely, an orientation in which an angle between a C-axis and a tangent on the surface becomes maximum, an M-axis orientation or an A-axis orientation is generally selected. These orientations can provide a high symmetry (i.e., atoms uniformly arranged) for the crystal and, therefore, they are selected to have steps with high linearity.
In order to achieve the above step-flow mode growth, a SiC substrate has been suggested which has a surface inclining in the range of 0.02° to 0.6° to a (0001) plane of the SiC crystal and having an inclination orientation within 7° from an M-axis orientation (See, e.g., JP-A-11-233391).
Furthermore, a GaN substrate has been suggested which has a surface inclining in the range of 1° to 20° to a (0001) plane of the GaN crystal and having an inclination orientation within 7° from an A-axis orientation (See, e.g., JP-A-2002-16000).
However, a practical device using the conventional substrate with the inclined surface and having a nitride semiconductor layer formed on the substrate still has various drawbacks as below which may narrow the application range of a nitride semiconductor device.
For example, a nitride semiconductor light-emitting diode currently in practical use causes a low color purity, i.e., a large half-value width in emission spectrum. Although this may not be always problematic for use in display, it will be very problematic for use in, e.g., a backlight of a liquid crystal display. Namely, in a liquid crystal display using a light-emitting diode, light emitted from the light-emitting diode is directed on its screen while being passed through an optical filter, so that light beyond the pass band of the optical filter is wasted by being blocked by the optical filter, which causes an increase in power consumption when intending to provide a necessary brightness for the screen.
Also in case of a nitride semiconductor laser diode, due to the inferior color purity of a light-emitting layer, threshold current for laser oscillation must be increased.
In a nitride semiconductor electronic device such as a high-electron-mobility transistor (HEMT), a phenomenon called “current collapse” occurs in which amplification varies when turning on electricity. Therefore, in fact, a great deal of effort has been made for suppressing the phenomenon over the crystal growth and the device fabrication.
In addition, non-uniformity in crystal growth of substrate is a serious problem for fabricating the above devices. For example, even when a particular region in plane of the substrate surface provides satisfactory characteristics for a device fabricated thereon, the other region in plane thereof frequently provides unsatisfactory characteristics for a device fabricated thereon.